In most nuclei and cortices, one can classify neurons into two broad categories: projection cells and local interneurons. Projection cells are neurons that send their axon to another region and thus are the origins of the various tracts of the nervous system. In contrast, local interneurons have axons that terminate in the same neural structure as their cell of origin and are involved with local computations rather than conveying signals from one region to another. These categories are not exclusive; many neurons have axons that both give off local branches and project to one or more distant regions.
• Fig. 4.7 Lateral view of the human brain showing the left cerebral hemisphere, cerebellum, pons, and medulla. Note the division of the lobes of the cerebrum (frontal, parietal, occipital, and temporal) and the two major fissures (lateral and central). (From Nolte J, Angevine J. The Human Brain in Photographs and Diagrams. 2nd ed. St Louis: Mosby; 2000.)
Regional Anatomy of the CNS
The spinal cord can be subdivided into a series of regions (see
Fig. 4.6A ), each composed of a number of segments named for the vertebrae where their nerve roots enter or leave: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each portion maintains its tubular appearance, although its lumen, the spinal canal, may not remain patent. Within the gray matter the dorsal horn receives and processes sensory information from the dorsal roots, whereas the ventral horn is primarily a motor structure and contains the motor neurons whose axons project out via the ventral roots (
Fig. 4.8
The surrounding white matter consists of many tracts interconnecting spinal cord levels and for communication with the brain. Three major ones are the lateral corticospinal tract (motor), spinothalamic tract/anterolateral system (sensory), and dorsal column-medial lemniscus pathway (sensory) (see
Fig. 4.6F
The brainstem consists of the medulla, pons, and midbrain (
Fig. 4.9
Fig. 4.6 ). In addition to the longitudinal pathways interconnecting with the spinal cord, the brainstem contains nuclei and many additional pathways that vary by level. These structures have many functions, some of which are analogous to those of the spinal cord (e.g., conveying basic sensory information and motor commands) and others related to a variety of other brain functions, such as cardiac control and state of consciousness. The brainstem also receives input and sends motor output via cranial nerves (
Table 4.1
The cerebellum sits dorsal to the pons and medulla. It receives inputs from spinal cord, brainstem, and cerebral cortex and projects back to many of these same structures. The cerebellum is critical for motor coordination but is increasingly recognized as having key roles in other cognitive functions.